1. Field of the Invention
The present invention relates to a blood pressure monitor apparatus which monitors blood pressure values iteratively obtained from a living subject.
2. Related Art Statement
A blood pressure monitor apparatus is used, for instance, for continuously monitoring the blood pressure of a patient or a living subject during an operation. One example of the blood pressure monitor apparatus is adapted to judge the abnormality of blood pressure of the subject when the blood pressure is higher than a predetermined upper threshold or lower than a predetermined lower threshold. Another example of the blood pressure monitor apparatus is adapted to determine, as a standard blood pressure of the subject, a blood pressure measured at a certain time point, and judge the abnormality of the blood pressure of the subject when an amount or a rate of change of the blood pressure of the subject from the standard blood pressure does not fall in a prescribed reference range. For instance, JP-A-11-318837 or its corresponding U.S. Pat. No. 6,027,455 discloses a technique to judge the abnormality of blood pressure of the subject, wherein 1) an estimated blood pressure is iteratively determined based on pulse-wave-propagation-velocity-related information which is related to a velocity at which a pulse wave propagates in the subject, and the abnormality of blood pressure of the subject is judged when the iteratively determined estimated blood pressure does not fall in a prescribed alert range, and 2) a blood pressure obtained at a prescribed period by using a cuff is determined as a standard blood pressure, and the abnormality of blood pressure of the subject is judged when a rate of change of the iteratively determined estimated blood pressure from the standard blood pressure does not fall in a prescribed reference range.
Where the abnormality of blood pressure of the subject is judged when the blood pressure is higher than the prescribed upper threshold or lower than the prescribed lower threshold, the abnormality of blood pressure is not found so long as the changed (increased or decreased) blood pressure falls within a normal or permissible range defined by the upper and lower thresholds, even if the change (increase or decrease) of the blood pressure may be large. In this case, an abrupt or rapid change of the physical condition of the patient may not be quickly detected. Where the abnormality of blood pressure is judged when the amount or rate of change of the blood pressure of the patient from the standard blood pressure does not fall in the prescribed reference range, the blood pressure of the subject may be judged to be abnormal even if the blood pressure may fall in the normal or permissible range. In this arrangement, however, the blood pressure of the patient may be judged to be abnormal even if the change of the blood pressure from the standard blood pressure may be toward a normal value. For instance, where the standard blood pressure is as low as the lower threshold of the normal range, it is not necessary to judge the abnormality of blood pressure even in the event of a great increase of the blood pressure. On the other hand, where the standard blood pressure is as high as the upper threshold of the normal range, it is not necessary to judge the abnormality of blood pressure even in the event of a great decrease of the blood pressure. However, the conventional blood pressure monitor apparatus described above judges that the blood pressure is abnormal when the amount or rate of change of the blood pressure from the standard blood pressure does not fall in the prescribed reference range, irrespective of the magnitude of the standard blood pressure. Thus, the conventional blood pressure monitor apparatus may judge the abnormality of blood pressure even in the above-described cases where a great increase or decrease of the blood pressure is not abnormal.